1. Field of the Invention
The present invention relates to a double frequency converter for use in television receivers or television signals transmitters in CATV systems and the like to convert the frequency twice.
2. Description of the Related Art
A double frequency converter according to the prior art for use in television signals transmitters in CATV systems, for instance, will be described with reference to FIG. 4.
Intermediate frequency television signals (the frequencies according to the U.S. specifications are 45.75 MHz for video intermediate frequency signals and 41.25 MHz for audio intermediate frequency signals) generated by a modulator (not shown) are entered into a first mixer 22 via an input filter 21. In the first mixer 22, they are mixed with first local oscillation signals entered from a first local oscillator 23 to be frequency-converted into first intermediate frequency signals of approximately 1300 MHz. If the frequency of the first local oscillation signals is 1254.25 MHz, for instance, those of the first intermediate frequency signals will be the respective differences from that, i.e. 1300 MHz for first video intermediate frequency signals Ifp and 1295.5 MHz for first audio intermediate frequency signals Ifs.
The first intermediate frequency signals, after being amplified appropriately, are entered into a band pass filter 24. The band pass filter 24 consists of a dielectric oscillator, designed to have a flat pass band of 4.5 MHz with a center frequency of 1297.75 MHz. The first intermediate frequency signals cleared of unnecessary signals outside the band by the band pass filter 24 are entered into a second mixer 25, wherein they undergo frequency conversion into television signals for use in usual broadcasting. Therefore, their frequency differs with the channel over which they are transmitted. For this reason, a second local oscillation frequency entered from a second local oscillator 26 into the second mixer 25 differs with the channel for which the conversion is to take place, and is controlled by a PLL circuit 27 to be variable between approximately 1350 MHz and 2350 MHz.
If the signals are to be transmitted over channel 2 (having a video carrier of 55.25 MHz and an audio carrier of 59.75 MHz), for instance, the second local oscillation frequency will be 1355.25 MHz, or if transmitted over channel 94 (having a video carrier of 643.25 MHz and an audio carrier frequency of 647.75 MHz), it will be 1943.25 MHz.
Television signals supplied from the second mixer 25, mixed with other television signals, are transmitted via a wide-band output filter 28.
Incidentally, although the band pass filter 24 is designed to have a center frequency F0 of 1297.75 MHz and a bandwidth of 4.5 MHz as represented by a solid line A in FIG. 5, dimensional fluctuations during the manufacturing process may invite a downward deviation of the center frequency f0 as shown by the dotted line B in FIG. 5. Where such a band pass filter is used, transmission will take place with the level of the video intermediate frequency signals Ifp kept down by X(dB) though the level of the first audio intermediate frequency signals Ifs remains unchanged, giving rise to a problem of deteriorated video quality at the receiving end.
Or, in order to avert this problem, it is conceivable to widen the pass band to prevent, even if the center frequency deviates, the levels of the first audio intermediate frequency signals Ifs and of the first video intermediate frequency signals Ifp from dropping, but this would give rise to another problem of C/N deterioration.
In view of these problems, the double frequency converter according to the present invention is intended to keep the bandwidth of the band pass filter narrow to prevent C/N deterioration, and yet to maintain the video carrier and the audio carrier on the final output channel at the same level even if the center frequency deviates from the design value.
In order to solve the above-noted problems, a double frequency converter according to the present invention is provided with a first mixer for frequency conversion of input signals with first local oscillation signals into first intermediate frequency signals; a second mixer for frequency conversion of the first intermediate frequency signals into output signals with second local oscillation signals; and a band pass filter disposed between the first mixer and the second mixer, wherein the frequency of the first local oscillation signals and the frequency of the second local oscillation signals can be shifted by the same frequency.
A double frequency converter according to the invention may as well be configured so as to alter the frequency of the second local oscillation signals at prescribed intervals, and to shift each frequency during the prescribed intervals.
Alternatively, a double frequency converter according to the invention may also be configured so as to alter the frequency of the first local oscillation signals at prescribed intervals, and to shift each frequency during the prescribed intervals.
Further, a double frequency converter according to the invention may as well be provided with a first PLL circuit for controlling the frequency of the first local oscillation signals; a second PLL circuit for controlling the frequency of the second local oscillation signals; and a microcomputer for supplying the first PLL circuit or the second PLL circuit with first frequency data for altering the frequency of the first local oscillation signals or the second local oscillation signals at the aforementioned prescribed intervals, wherein second frequency data based on the frequency of the deviation between a reference frequency of the first intermediate frequency signals and the center frequency of the band pass filter is stored into the microcomputer, and shifting is accomplished by supplying the second frequency data that has been stored to the first PLL circuit and the second PLL circuit.
In a double frequency converter according to the invention, the first PLL circuit and the second PLL circuit may have minimum step frequencies smaller than the frequency of the prescribed intervals, using, as reference for the second frequency data, a frequency equal to an integral multiple of the minimum step frequency closest to the frequency of the aforementioned deviation.